In conventional terephthalic acid (TPA) or isophthalic acid (IPA) production processes, p-xylene or m-xylene, respectively, undergoes oxidation in the presence of a catalyst and an organic solvent, typically acetic acid, to form a crude terephthalic acid (CTA) or a crude isophthalic acid (CIPA) slurry (collectively referred to herein as a crude carboxylic acid or CCA). The CCA slurry can be further crystallized if desired to improve downstream solid/liquid separation. The TPA or IPA solids in the CCA slurry withdrawn from the primary oxidation reaction vessel or from a crystallization zone are concentrated or isolated in a solid/liquid separation device and washed with a wash organic solvent to remove most of the catalyst, thereby generating a CCA wet cake composition. The CCA wet cake composition is then typically dried to remove any remaining organic solvent such as acetic acid, after which it is slurried in water, and subsequently subjected to catalytic hydrogenation to form a purified terephthalic acid composition (PTA) or a purified isophthalic acid composition (PIPA) (collectively referred to as PCA).
Instead of hydrogenation or in addition to hydrogenation, a slurry of CCA particles withdrawn from the primary oxidation reactor can be subjected to secondary or post oxidation processes to form a PCA composition. It is also desirable to remove as much catalyst as possible for recovery and/or recycling to the primary oxidation zone. Such catalyst recovery can occur prior to entry into or subsequent to the secondary or post oxidation reaction zones.
In a hydrogenation reaction, it is desirable to conduct an organic solvent swap to exchange the organic solvent in the CCA composition for water prior to hydrogenation. In addition, the catalyst contained in the CCA composition is also removed to recover the catalyst and optionally recycle the catalyst to the oxidation zone.
In preparation for hydrogenation, or prior to or after secondary oxidation, the CCA composition is isolated for treatment. Various techniques are available for isolating a CCA composition. An example of one such technique includes the use of batch or continuous vacuum filters, batch or continuous pressure filters, centrifuges and the like. In solid/liquid separation devices, CCA solids are formed into a wet cake and subjected to one or more washes with a wash organic solvent. The purpose of the wash is to minimize the amount of catalyst retained in the CCA wet cake composition 3.
It is desirable to remove as much catalyst as possible from the CCA wet cake composition 3 to have high recovery of catalyst and/or maximize the amount of catalyst that can be recycled to the oxidation zone, as well as reduce the amount of catalyst left in the filter cake which can be detrimental in a hydrogenation reaction or in the process for making reaction products of PCA such as polyesters. In one purification technique, the filter cake is slurried in water and fed to the hydrogenation zone as a slurry. Catalyst retained in the filter cake can carry over into the hydrogenation zone and foul or reduce the activity of the hydrogenation catalyst bed.
In the case that one employs a hydrogenation step, it also becomes necessary to swap the organic solvent in the CCA composition with water. Washing a wet filter cake or a concentrated slurry of CTA and or IPA with water results in the mixing of water with organic solvent that, if the concentration of organic solvent is sufficiently high, would require an organic solvent recovery step before the water can be fed to a waste water treatment facility.
To further ensure that all the organic solvent is removed from the CCA prior to hydrogenation, the wet cake is usually dried before forming a water based slurry of TPA or IPA as a feed to the hydrogenation zone. A drying step, however, requires the input of energy.
It would be desirable to remove as much catalyst as possible in a CCA composition. Alternatively, it would be desirable to exchange the organic solvent with water in a way that reduces the concentration of organic solvent present in the waste water such that a separate organic solvent removal step can be avoided. Alternatively, it would be desirable to provide a process that can avoid, if desired, a drying step before creating a water slurry feed to the hydrogenation reactor. Depending on the purification technique used, it would be ideal if any combination of two or more of these advantages could be realized.